1. Field of the Invention
The present invention relates to a ball screw mechanism composed of a screw shaft with a male screw groove formed on an external surface thereof, a nut with a female screw groove on an internal surface thereof, plural balls held in the both screw grooves of the screw shaft and the nut, and a ball circulation piece fit in a window portion formed in the nut for circulating the balls. It also relates to a machining method for a joint between the nut and the ball circulation piece and further to an electric power steering device employing the ball screw mechanism.
2. Discussion of the Related Art
In an electric power steering device (hereafter referred to as “EPS”) of a lack assist type, a ball screw mechanism is used for transmitting the rotational power of an electric motor to a lack shaft. The ball screw mechanism used in the EPS is constituted as of an internal circulation type wherein plural balls are circulated inside the nut as described in Japanese unexamined, published patent application No 11-270648.
In the ball screw mechanism of the internal circulation type, as shown in FIG. 2, a male screw groove 16 and a female screw groove 22 along which plural balls 21 are guided and rolled are helically formed on a lack shaft 11 and a nut 20, respectively. Each time moving along the female screw groove 22 through one turn, each ball 21 is led to a circulation groove 24 which is formed on a ball circulation piece 25 secured to the nut 20 for bridging adjoining groove segments or portions in the axial direction of the female screw groove 22, and is circulated by overleaping a screw thread which is formed between adjoining groove portions in the axial direction of the male screw groove 16.
The aforementioned ball circulation piece 25 fixed to the nut 20 is made separately from the nut 20 and is integrated with the nut 20 by being fit in a window portion of an elongate hole, which is formed to bridge the adjoining groove portions of the female screw groove 22 of the nut 20. Because of errors in machining and assembling, simply fitting the ball circulation piece 25 in the window portion of the nut 20 causes a step to be generated at each of joints between the circulation groove 24 and the female screw groove 22. As being caught by the step, the balls cannot move smoothly, which causes the generation of noise and vibration.
For this reason, it has been a practice to perform a machining for eliminating the steps at the joints between the circulation groove 24 and the female screw groove 22. FIGS. 7 and 8 show one of joint portions or areas (E) between the circulation groove 24 and the female screw groove 22 in an enlarged scale. As shown by the solid line, the joint (A) has the step thereat. Therefore, heretofore, the step at the joint (A) has been eliminated by using a ball end mill (K) having a cutting portion of a diameter equal to or larger than that of the balls 21. That is, as shown in FIG. 8, the ball end mill (K) is infed into the female screw groove 22 by a target infeed depth (t) which is necessary to eliminate the step at the joint (A) and then is moved toward the circulation groove 24 along the center line of the female screw groove 22 thereby to machine the joint area (E) between the female screw groove 22 and the circulation groove 24.
However, when the joint (A) is machined with the ball end mill (K) given the target infeed depth (t), the ball end mill (K) is sharply infed at an infeed starting position. As the machining starting portion is viewed from over the female screw groove 22 as shown in FIG. 7, boundary lines (D) between a finished surface (B) and an unfinished surface (C) lie to extend laterally from a groove bottom toward flanks at both sides of the female screw groove 22, and the angle (α) made by the boundary lines (D) becomes an obtuse angle. Where the angle (α) made by the boundary lines (D) becomes such an obtuse angle, the contact points of each ball 21 with the female screw groove 22 changes abruptly at the boundary lines (D) when each ball 21 moves within the joint area (E) between the circulation groove 24 and the female screw groove 22. The abrupt change of the contact points of each ball 21 with the female screw groove 22 undesirably causes noise and vibration to be generated during the operation of the ball screw mechanism.